The WHO who SAID, like, THAT. Yeah.

What would happen if an Adventurer of D&D is summoned in the Grail War?

Chaos, that is.
_______________________________

True Name: ??
Class: 'Caster'

STR E
AGI E
END C
MANA C/EX
LUCK A
N.PHANTASM ??

Class Skills: Territory Creation C, Item Creation D+++

Skills: Vancian Casting C, Psionics A+++, Librarian of Stored Knowledge B

Noble Phantasm: Essence A, Spoils of War A, Legacy of Lost Items and Events EX

Territory Creation C
Due to Caster wandering nature, the ability to make a fixed stronghold is somewhat limited for it's class. It can supplement it with enough time and preparation, but the options are somewhat limited.

((Use of Permanency, Transmutation Spells, Illusions and Conjurations to create a good enough stronghold, while abusing the Auto-Resetting Magical traps rule to hell and back. It's level is actually tied to Vancian Casting stat due to access of higher level spells.))

Item Creation D+++
Due to Caster wandering nature, the ability to create powerful magic items are limited. However when certain factors are meet the item creating potential is enhanced.

((This Caster have Scribble Scroll, Craft Wondrous Item and Craft Universal Item featsl, but that's more than enough. Again, the real limiter is the access to higher level spells/powers and different enough powers. Of course, there are also Auto-Resetting Magic/Psionic traps you can use and abuse, but are harder to quantify.))

Vancian Casting C
A special form of magic native to Caster's world. Though with less overall power for it's supposed level of complexity, in certain matters their effects are absolute and inviolable, enforced by Akasha itself.

((Access to 5th level spells. Unfortunately most of the broken spells are higher level, but things like Protection from Evil are absolute in their effects, and nothing less than direct divine intervention with specific abilities or epic spells can bypass it, other than just old plain dispelling it.))

Psionics A+++
A special form of mystical power derived from the mind, sometimes incompatible with classical magecraft. The highest possible level of mental might one might attain before ascending to truly higher levels of existence.

((Access to 9th level powers? Check. Beating up action economy silly? Check. Doing that almost for free? Check, check, check. The only reason this doesn't get EX is because it is pre-epic and have limited access to powers.))

Expert of Many Specializations A
Caster wandered to many places and learned many things. Caster have access to several skills, ranging from E-rank to A-rank. Allows to identify

((Many knowledge skills, spellcraft, psicraft, some craft skills, a few odd skills such as balance and tumble. Other skills might or not be there))

Essence, Support, Self, A
The individual essence, basically the life force of one being. Caster can use the very essence of self to use certain abilities and create magical items. Sometimes used as currency with extremely powerful entities.

((Basically, XP, more specifically the halfway amount of XP between level 20 and 21. The reason it can be used as currency comes in the form of the Trap the Soul and similar spells, who in turn are traded later for items or even used as magical components for spells and rituals, alongside with trading with other high level souls on the afterlife between gods, demons and devils. Do you really think people walk around with 200k in cash even on the City of Doors?))

Spoils of War, Support, Self, A
The ability to claim the equipment worm and held of a fallen enemy. So long as Caster was directly involved in the defeat of an enemy, Caster can claim ownership of the enemy items regardless of rank, who can later be redistributed by Caster's discretion. This possession doesn't translate to skill in the item use, but only that Caster can use them. Eventually.

((The natural tendency of an adventurer to loot everything, from the enemy items to the enemy itself. Destroying magical items for an adventurer for any and all reasons are usually considered taboo. Caster also have access to Use Magic Device and Use Psionic Device, which translates to being able to activate certain Noble Phantasms))

Legacy of Lost Items and Events, Support, Self, EX
As an adventurer, Caster collected many treasures of many worlds, and eventually stumbled over tablets of lost and forbidden knowledge, alongside with powerful artifacts both Holy and Profane, some considered unspeakable Abominations even from Devils and Demons. Of course Caster keep all of them, a few of them at arms reach.

((Items of +6 to stats, all +5 books, a small armory of +x weapons, armors and shields of all times. Rings of Three Wishes, Candles of Invocation and basically any non-epic level item a DM would just laugh and throw the DMG at a player's head. Some are customized, like the Ring of Spellcraft +30, while others are sought after by anything with half a brain, such as Thought Bottles. Name it, Caster have at least one copy or know where to find it. A few of those items are even on Caster possession right now. Of course this also translates into certain levels of cheese, such as action economy abuse, NI loops, level drain + restoration abuse, wish based economy, The Wish/The Word, chaos shuffle, dragonwrought kobold with access to epic spells and others. Fortunately the true broken stuff like Omniscifer and Pun-Pun are held back in a treatise enforced by between multiple overdeities, Pun-Pun included, sealed and guarded by the Lady of Pain.))